A. On the third floor of a parking garage near Miami State University.

Q.
When did we see it on the show?

A. In Episode 9, of Season 7, "Helter Skelter".

Dexter gets a call from Angel that a second arson victim has been found in an elevator. (The first victim was found in a burned-out car.)

When he arrives, Dexter finds a badly burned corpse slouched in the corner of an elevator.

Quinn says the victim was a grad student.

The same fire inspector is there, saying that this time it doesn't look like a suicide.

Dexter
asks them to close the elevator doors, and when they do, they see a
reverse shadow on the inside doors, as if someone was standing there,
wearing a fire-proof suit, blocking the exit and watching the victim
burn alive.

They also see the word "Bobby" written in the ashes on the door.

Back
outside, Dexter takes Debra aside and tells her that he thinks that
Hannah is being held hostage at the home of one of the Columbian
victims of the shooting at Mateo's Bar.

Dexter
spots one of the assassins (who are after Isaak) following him. Dexter phones
Isaak and lets him know. Isaak tells him to lead the guy to a cargo ship,
the Fearless, at Pier 18.

Q.
What is it actually in real life?

A. An elevator in a parking structure.

Q.
Where can I find it in real life?

A. This scene was shot on the third level of a parking structure at the Long Beach Airport Business Park, at 5000 E. Spring Street, in Long Beach, CA.

That's just north of the Long Beach Airport, and just south of the Skylinks Golf Course.

In the scene, when you see the striped building outside the garage, you're looking northeast, towards the office building that houses P2S Engineering.

Here is a screencap from the scene, with most of the photo darkened to highlight the view outside:

A. In the early seasons, I usually had to hunt down the locations after viewing the episodes,
using clues from the various scenes. By the now, though,
I've developed a small group of fans, spies & tipsters who kept an
eye out for Dexter filming in their neighborhoods, and would let me know
in advance when something was about to film there.

Between those reports, my own personal reconnaissance around town, and a
few new resources I discovered, by the time the first episode airs, I
already know most (but not all) of the filming locations, and only
need to watch the episodes and match up the scenes with the correct
locations.

This group of helpful fans includes Rick, Ellen, Elaine, Mia, Joel, Susan, Eric, Jeff, Kerry & others. My thanks to all of
them.

This tip came from Rick.
And it was easy to confirm by comparing the view seen outside
the parking building to the aerial map photos of the office park.
(Thanks, Rick!)